Three phase light source for pinhole detector



July 30, 1968 D. R. BROSIOUS ET AL 3,395,286

THREE PHASE LIGHT SOURCE FOR PINHOLE DETECTOR Filed Nov. 17, 1964INVENTORS Daniel R. Bros/bus James K Holl/hgsheaa United States Patent3,395,286 THREE PHASE LIGHT SOURCE FOR PINI-IOLE DETECTOR Daniel R.Brosious, Bethlehem, and James K. I-Iollingshead, Hellertown, Pa.,assignors, by mesne assignments, to Bethlehem Steel Corporation, acorporation of Delaware Filed Nov. 17, 1964, Ser. No. 411,822 2 Claims.(Cl. 250219) ABSTRACT OF THE DISCLOSURE Three parallel fluorescent tubesdefining apices of an equilateral triangular prism are nested in areflector with the base of the prism parallel to and aligned over theslot of the pinhole detector, each tube being operated from one phase ofa three phase AC power supply.

This invention relates generally to apparatus for detecting pinholesand, specifically, to apparatus for accurately counting pinholes instrip which may be moving past the apparatus at speeds up toapproximately 5000 feet/minute.

In the manufacture of tinplate and strip, pinholes, which may have adiameter of as little as approximately one mil (0.001 inch), are formedas a result of small steel or non-metallic particles being rolled intothe strip during the rolling operation, which particles subsequentlyfall out. The quality of the strip is in part determined by the numberof pinholes per unit length, it being apparent that the higher thequality of the strip, the fewer will be the number of pinholes per unitlength. (Strictly speaking, the term pin'hole-feet/ 1000 feet isemployed in the art, but this does not affect the description of thepresent invention.) Therefore, to determine whether or not a length ofstrip meets specifications, the number of pinholes must be counted.Pinholes are conventionally detected at production speeds by passingstrip between a light source and a photosensitive tube or tubes, thelatter being mounted in a chamber and adapted to receive only lightpassing through the said pinholes. When a pinhole passes between thelight source and the photosensitive tube, the latter generates anelectrical signal indicative of the presence of the pinhole. It isimportant that the light source be of substantially constant intensityfor two reasons. Firstly, when the strip passes between the light sourceand the photosensitive tube or tubes at high speeds, any flickering orpulsation in the intensity of the light source might well result in thepassage of a pinhole without detection thereof. Thus, where the slot inthe chamber exposing the photosensitive type or tubes may be 1% inchesin width, and the speed of the strip may be 5000 feet/minute, a pinholein the strip would traverse the slot in approximately 0.00125 second andlight sources operating on single-phase 60 cycle current, or even onthree-phase 60 cycle current in apparatus not employing the geometry andlighting levels of the present invention, would not count all of saidpinholes. A second reason of considerable importance in respect to therequirement of substantially constant light intensity occurs when thestrip has been stopped or is travelling past the slot at very lowspeeds, say 5-6 feet/minute. Any fluctuation or pulsation in theintensity of the light source While the pinhole is over the slot mightinclude excursions of light intensity alternately below the thresholdlevel of the photosensitive tube or tubes and above the said threshholdlevel. Obviously, the photosensitive tube will respond to lightintensities below its threshold level as if no light "ice at all werestriking the said photosensitive tube or, in other words, as if thepinhole had already passed, and a subsequent rise in intensity of lightabove the threshold level, as part of such regular cyclic fluctuationsor pulsations, would erroneously be interpreted by the photosensitivetube as another and separate pinhole. Thus, as long as the pinhole isover the slot in the chamber, which may be for an indefinite period whenthe strip is stopped, or which may for instance be for 1.25 seconds whenthe strip travelling 5 feet/minute past a 1% inch slot, thephotosensitive tube would erroneously count pinholes at the rate of persecond when the light source is powered by single-phase 60 cycle currentand also at a fictitious rate when the light source is powered bythree-phase 60 cycle current in apparatus not employing the geometry andlighting levels of the present invention. It is clear that, when thestrip has stopped with a pinhole over the slot, the photosensitive tubeor tubes will erroneously count pinholes indefinitely and, where asingle pinhole traverses the 1% inch slot at 5 feet/minute, under asingle-phase 60 cycle-powered light source, the photosensitive tube ortubes will erroneously count pinholes, for an error of 14,900 percent.

The foregoing problems have not gone unrecognized in the art.Heretofore, light sources for the above-mentioned application comprisedone or more fluorescent lamps energized by direct current or byalternating current of high frequency (frequencies above 60 cycles persecond). Even so, such light sources were quite complex, requiredfrequent maintenance and were difficult to regulate due, among otherreasons, to the negative resistance characteristics of gaseous dischargetubes as known to those familiar with this art. Moreover, light sourcespowered by direct current were subject to degradation due to themigration of mercury within the fluorescent tube resulting in thedarkening of the ends of said tube, and attempts to minimize mercurymigration by frequent reversal of polarity have not always beensuccessful.

One of the objects of this invention is to provide apparatus fordetecting pinholes.

Another object of this invention is to provide apparatus for accuratelycounting pinholes in strip which may be stopped or moving at productionspeeds past the said apparatus.

A further object of this invention is to provide for apparatus fordetecting pinholes a substantially constant intensity light sourcepowered by three-phase 60 cycle current, which light source is simple indesign and easy to maintain.

Other and further objects of this invention will become apparent duringthe course of the following description and by reference to theaccompanying figures and the claims appended hereto.

Broadly, the present invention is directed to apparatus for detectingpinlidle's in strip adapted to move past a slot in a chamber of theapparatus containing a photo-multiplier tube, with a particular lightsource on that side of the strip opposite the slot, which light sourcecomprises a plurality of fluorescent tubes or lamps nested within areflector, each fluorescent tube or lamp being powered by one phase of apolyphase source of current. Substantially continuous constant levelillumination for pinhole detection is obtained from the combination ofthe separate single-phase-powered fluorescent tubes or lamps operated insequence from the polyphase source of power and the particulargeomet-fyand lighting levels associated with the fluorescent tubes, thesteel strip and the photomultiplier tube. a

Referring now to the drawings, in which like numerals represent likeparts in the several views:

FIGURE 1 represents an enlarged section in elevation of a portion ofstrip with a pinhole extending therethrough and showing in dashed linesthe light-intercepting cone passing therethrough.

FIGURE 2. represents a view in elevation of the pinhole detectionapparatus including the light source, and showing in dashed lineslight-intercepting cones corresponding with several successive positionsof a pinhole as it moves past the slot in the light detection chamber.

FIGURES 3a-3e represent diagrammatically several light interceptionpatterns seen by the photomultiplier tube in the light detection chamberand corresponding with the successive positions of the pinhole shown inFIGURE 2.

The present invention, in the preferred embodiment hereinafterdisclosed, is designed primarily for the detection of pinholes which areassumed to be approximately circular and about 0.001 inch in diameter orslightly larger in strip which may range between approximately 0.006inch and 0.0167 inch in thickness. Taking a rough average stripthickness of 0.010 inch, it can be shown that light passing through a0.001 inch diameter pinhole in such strip will form a cone having anapex angle of about ll.4, neglecting internal reflections in thepinhole, as illustrated in FIGURE 1. This ll.4 cone is the basis for thesuccessive light intercepting patterns shown in the other figures.Notwithstanding the foregoing, however, the specific 11.4 cone referredto is a function of strip thickness and hole size, and is illustrativeonly. The criterion for the successful practice of this invention isthat the fluorescent tubes or lamps be arranged geometrically and insuch an array and in such proximity to each other and in such positionrelative to the strip that, at any position of the pinhole relative tothe slot of the light detection chamber, a cone having its apexsubstantially in the center of the pinhole (i.e., on the longitudinalaxis of the hole midway through the strip) will intercept at least twoof the fluorescent tubes or lamps.

The pinhole detecting apparatus comprises light source 1 and lightdetection chamber 2, between which strip 3 is passed.

Referring to FIGURE 2, strip 3 having a pinhole 4, traverses slot oflight detection chamber 2, in closely adjacent relation thereto tominimize or eliminate light entering the said light detection chamber 2from any source or direction other than through the said pinhole 4.Strip 3 passes slot 5 from left to right of FIGURE 2, as shown by thearrow, and five successive positions of pinhole 4 are indicated as 4a,4b, 4c, 4d and 4e. The 11.4 light cones corresponding to each of thefive successive pinhole positions shown in FIGURE 2 are indicated bydashed lines. In one construction of the present invention, slot 5 is 1%inches in width (the dimension shown in FIGURE 2).

Light detection chamber 2 contains a conventional photomultiplier tube,not shown, connected to circuitry which counts the number of pinholes 4(strictly speaking, pinhole-feet/ 1000 feet) traversing slot 5, all ofwhich is known to those familiar with this art. For the purpose of thisspecification, it will be observed that the photomultiplier tube, whichmay for example be of the type known in the art as #931, produces anelectrical output in response to the level of illumination interceptedby the said tube, the output rising with increase in the level ofillumination and then levelling off as saturation is approached. Thereis a threshold of sensitivity of the tube below which substantially noelectrical output appears; thus, with levels of illumination below thisthreshold of sensitivity, the tube will repsond as if it were in totaldarkness.

In the preferred embodiment, light source 1 comprises three fluorescenttubes 6, 7 and 8 mounted within an integrating reflector 9, and oflength suflicient to extend from side to side of strip 3. Each of thefluorescent tubes 6, 7 and 8 is powered by one phase of a standardthreephase 60 cycle power supply and therefore the current passingthrough said tubes 6, 7 and 8 will be 120 out of phase. Thus, if thetubes 6, 7 and 8 shown in FIGURE 2 are connected to be sequentiallypowered in a clockwise direction, tube 7 will reach maximum brightness120 or A of a second after tube 6 reaches maximum brightness; tube 8will reach maximum brightness 120 or ,6 of a second after tube 7 reachesmaximum brightness; and tube 6 will reach maximum brightness or ,5 of asecond after tube 8 reaches maximum brightness. It will be understoodthat tubes 6, 7 and 8 as shown in FIGURE 2 can also be connected to besequentially powered in a counter-clockwise direction.

Tubes 6, 7 and 8 are arranged with respect to each other and withrespect to strip 3 and slot 5 so that an 11.4 cone having its apex atthe center of pinhole 4 as shown in FIGURE 1 will intercept at least twoof the three said tubes at all times when pinhole 4 is traversing slot5, whereby, as hereinafter explained, the level of illuminationperceived by the photomultiplier tube in light detection chamber 2 willalways remain above the threshold level of said photo-multiplier tubewhen said pinhole 4 is traversing slot 5.

Thus, as shown in the preferred embodiment, tubes 6, 7 and '8 arearranged as apices of a horizontal equilateral triangular prism inreflector 9, the bottom face of the triangular prism as defined inFIGURE 2 by tubes 6 and 8 being parallel to strip 3 and the verticalaxis of the triangular prism as shown in FIGURE 2 overlying the centerof slot 5. Tubes 6, 7 and 8 are spaced from each other, as vibrations inthe strip mill or tin mill in which the present invention is primarilyintended for use could shatter the said tubes if they were in directphysical contact with each other and, more importantly, to provide atleast between tubes 6 and 8 a gap 10 exposing part of tube 7 tointerception by an 1l.4 cone at any position of pinhole 4 relative toslot 5. In one construction of the present invention employing tubes 6,7 and 8 of a standard 1 /2 inch O.D., the distance between tube centersis 1% inches, gap 10 is A inch, and the distance between the centers oftubes 6 and 8 and strip 3 is 4% inches.

Tubes 6, 7 and 8 are, of course, chosen to provide sufficientillumination so that as pinhole 4 traverses slot 5, the photomultipliertube is always illuminated above its threshold level. Specifically, whenpinhole 4 is in position 4a or closely adjacent thereto, so that verylittle or no part of tube 8 is intercepted by an ll.4 cone having itsapex in said pinhole 4, tube 6 should be capable independently ofilluminating through pinhole 4 the photomultiplier tube at least to itsthreshold level when tube 7 is dark, and that portion of tube 7 notblocked by tube 6 should also be capable independently of illuminatingthrough pinhole 4 the photomultiplier tube at least to its thresholdlevel when tube 6 is dark. Specifically, when pinhole 4 is in position4e or closely adjacent thereto, so that very little or no part of tube 6is intercepted by an 11.4 cone having its apex in said pinhole 4, tube 8should be capable independently of illuminating through pinhole 4 thephotomultiplier tube at least to its threshold level when tube 7 isdark, and that portion of tube 7 not blocked by tube 8 should also becapable independently of illuminating through pinhole 4 thephotomultiplier tube at least to its threshold level when tube 8 isdark. If these requirements are met, then at all positions of pinhole 4intermediate positions 4a and 4e, the photomultiplier tube will alwaysbe illuminated through the said pinhole 4 above its threshold level. Ofcourse, integrating reflector 9 may reduce these requirements, and somecredit may also be allowed to light from tube 7 filtering through tubes6 and 8 to reduce these requirements. As a practical matter, each oftubes 6, 7 and 8 should be capable independently of providingillumination well above the minimum levels and, in one construction ofthe present invention as herein before described and employing a #931photomultiplier tube, tubes 6, 7 and 8 are of 5 ft. lengths with 72watts rating. To demonstrate the foregoing, and by way of illustratingthe operation of this invention, reference is made to FIGURES 3a-3ewhich correspond with five successive positions of pinhole 4 in itspassage across slot 5.

FIGURE 3a shows the pattern of illumination seen by the photomultipliertube when pinhole 4 is in position 4a. The 11.4 cone intercepts tube 6and also part of tube 7 through gap 10. The diagonally shaded circularsegment at the left of FIGURE 3a corresponds with that portion of tube 6(also similarly diagonally shaded for ease of comprehension) interceptedby the 11.4 cone and represents the amount of light from tube 6 fallingon the photomultiplier tube when said tube 6 is lit. The verticallyshaded circular segment at the right of FIGURE 3a corresponds with thatportion of tube 7 (also similarly shaded for ease of comprehension)intercepted by the 11.4" cone and represents the amount of light fromtube 7 falling on the photomultiplier tube when said tube 7 is lit. Thevertically shaded circular segment is of greater vertical spread thanthe diagonally shaded circular segment because tube 7 is farther awayfrom pinhole 4 than tube 6. Tubes 6 and 7 will be passing from maximumpositive to maximum negative voltage through 0 voltage, 120 out ofphase, and their respective brightness will therefore vary. If tube 6 isabsolutely dark (i.e., going through 0 voltage) tube 7 will, ashereinbefore described, be capable independently of illuminating thephotomultiplier tube above its threshold level. As tube 7 becomesabsolutely dark, tube 6 will, as hereinbefore described, be capableindependently of illuminating the photomultiplier tube above itsthreshold level. At other times, the sum of illumination received fromtubes 6 and 7 will always maintain the photomultiplier tube above thethreshold. Thus, even if pinhole 4 remains in position 4a, once it hasbeen detected by the photomultiplier tube the level of illumination ofthe photomultiplier tube will remain above the threshold until thepinhole 4 passes slot 5, and the photomultiplier tube will count onlyone pinhole, rather than count indefinitely at a high rate as would bethe case if the geometry and lighting levels of the present inventionwere not employed. FIGURE 30 ignores the transmission of light from tube7 through tube 6 as well as the effect of the integrating reflector 9.

FIGURE 3b shows the pattern of illumination seen by the photomultipliertube when pinhole 4 has moved to position 4b. The 11.4 cone nowintercepts tubes 6 and 8 and tube 7 through gap 10. The diagonallyshaded circular segments at the left and right of FIGURE 3b correspondwith those portions of tubes 6 and 8 respectively (similarly shaded forease of comprehension) intercepted by the 11.4 cone and represent theamount of light from tubes 6 and 8 respectively falling on thephotomultiplier tubes when said tubes 6 and 8 are lit. As in FIGURE 30,the vertically shaded circular segment at the center of FIG- URE 3bcorresponds with that portion of tube 7 (also similarly shaded for easeof comprehension) intercepted by the 1l.4 cone and represents the amountof light from tube 7 falling on the photomultiplier tube when said tube7 is lit. Again, the vertically shaded circular segment is of greatervertical spread than either diagonally shaded circular segment becausetube 7 is farther away from pinhole 4 than tubes 6 and 7. If any of thetubes 6, 7 or 8 is dark (i e., going through 0 voltage), either of theother tubes will be capable independently of illuminating, throughpinhole 4, the photomultiplier tube above its threshold level, ashereinbefore described. At other times, when all three tubes 6, 7 and 8are lit and are various degrees removed from maximum brightness, thetotal illumination will always maintain the photomultiplier tube abovethe threshold. Again, even if pinhole 4 remains in position 4b, once ithas been detected by the photomultiplier tube in position 4a (FIGURE 3a)the level of illumination of the photomultiplier tube will remain abovethe threshold until 6 the pinhole 4 passes slot 5. In FIGURE 3b,transmission of light from tube 7 through tubes 6 and 8 as well as theeffect of the integrating reflector 9 have been ignored.

FIGURES 3c, 3d and 3e show the patterns of illumination seen by thephotomultiplier tube as pinhole 4 moves across slot 5. In all cases,once pinhole 4 has been detected by the photomultiplier tube (i.e., whenpinhole 4 is over slot 5), whether strip 3 is moving at high productionspeeds or whether it stops with pinhole 4 over slot 5 for anindeterminate period of time and then continues past slot 5, theillumination of the photomultiplier tube will remain above the thresholdlevel until pinhole 4 passes slot 5 and will not miss the said pinholenor erroneously count more than one pinhole.

In summary, we have provided apparatus for counting pinholes in strip,employing a light source comprising a particular geometry and lightinglevel of a plurality of fluorescent tubes or lamps nested within areflector, each fluorescent tube or lamp being powered by one phase of apolyphase source of current, whereby a true count of pinholes is made atproduction speeds of the strip, at very slow speeds of the strip, andwhen the strip has stopped with a pinhole over the apparatus.

While we have shown and described the best embodiment of our inventionnow known to us, we do not wish to be limited to the exact structureshown and described herein, but may use such substitutions,modifications and equivalents as are embraced within the scope of thespecification and drawings or as pointed out in the appended claims.

We claim:

1. In combination with pinhole detecting apparatus comprising a lightdetection chamber having a slot therein adapted to be traversed by astrip having a pinhole therethrough, a light source comprising:

(a) three fluorescent tubes, each powered from a different phase of a 3phase source of alternating electric current,

(b) said fluorescent tubes being arranged parallel to each other and tosaid slot and defining the apices of a triangular prism, one face ofsaid triangular prism being parallel to and facing said slot, said facefurther being centered over said slot, the apex of said triangular prismopposite said face being aligned with the perpendicular bisector of saidface, so that light from at least one of said fluorescent tubes passesthrough said pinhole into said light detection chamber at any positionof said pinhole over said slot, said fluorescent tubes being of suchsize that said light from any of said three fluorescent tubes is ofsufiicient intensity to actuate said pinhole detecting apparatus.

2. In combination with pinhole detecting apparatus comprising a lightdetection chamber having a slot therein adapted to be traversed by astrip having a pinhole therethrough, a light source comprising:

(a) a first fluorescent tube and two second fluorescent tubes, eachpowered from a dilferent phase of a 3 phase source of alternatingelectric current,

(b) said three fluorescent tubes being arranged parallel to each otherand to said slot and defining the apices of equilateral triangularprism, said second fluorescent tubes defining a face of said triangularprism centered over, facing and parallel to said slot, said secondfluorescent tubes being spaced from each other so as to provide a gaptherebetween, some portion of said first fluorescent tube being indirect line of sight of said pinhole through said gap at any position ofsaid pinhole over said slot so that light from said first fluorescenttube and at least one of said second fluorescent tubes is adapted topass through said pinhole into said light detection chamber at anyposition of said pinhole over said slot, said three fluorescent tubesbeing of such size that said light from any one of said threefluorescent References Cited UNITED STATES PATENTS 8/1951 Adams 315-144X 7/1954 Craig 315-444 X 8 Linderman 250-219 Dowell et a1 315-144 XGoodwin et a1 250-219 Shaheen 315144 X RALPH G. NILSON, PrimaryExaminer.

M. A. LEAVITT, Assistant Examiner.

